Organic electronic elements are elements intended for electrical operations with the use of organic matters, expected to be able to provide features such as energy conservation, low prices, and flexibility, and attracting attention as alternative techniques to conventional inorganic semiconductors mainly containing silicon.
Examples of the organic electronic elements include organic EL elements, organic photoelectric conversion elements, and organic transistors.
Among the organic electronic elements, the organic EL elements are attracting attention, for example, as alternatives to incandescent lamps and gas-filled lamps, and for use as large-area solid-state light sources. In addition, the organic EL elements are also attracting attention as most likely self-luminous displays in place of liquid crystal displays (LCD) in the field of flat panel display (FPD), and increasingly put into production.
In recent years, for the purpose of improving the organic EL elements in luminescent efficiency and lifetime, attempts have been made to use a charge transporting compound mixed with an electron-accepting compound.
For example, Patent Literature 1 discloses a composition composed of an ionic compound and a charge transporting compound, as a composition for charge transporting films.
For the purpose of improving the organic EL elements in luminescent efficiency and lifetime, attempts have been made to use a charge transporting compound mixed with an electron accepting compound.
For example, Patent Literature 1 discloses a composition composed of an ionic compound and a charge transporting compound, as a composition for charge transporting films.
As just described, it is considered important to produce a compound composed of radical cations of the charge transporting compound and counter anions, which is produced in the case of mixing the charge transporting compound and the electron accepting compound.
On the other hand, the organic EL elements are classified roughly into two types of: low molecular weight-type organic EL elements and high molecular weight-type organic EL elements, according to materials and film forming methods used. The high molecular weight-type organic EL element are essential elements to large-screen organic EL displays in the future, because organic materials are composed of high molecular weight materials, and able to be easily formed by printing, ink-jet printing, etc., as compared with the low molecular weight-type organic EL elements which require film formation in a vacuum system.
Both the low molecular weight-type organic EL elements and high molecular weight-type organic EL elements have been energetically researched, but still have the significant problem of being low in luminescent efficiency and short in element lifetime. As one means for solving this problem, multi-layered elements have been attempted for the low molecular weight-type organic EL elements.
FIG. 1 shows an example of a multi-layered organic EL element. In association with FIG. 1, a layer in charge of light emission is referred to as a light emitting layer 1, and in the case of including other layers, a layer in contact with an anode 2 is referred to as a hole injecting layer 3, and a layer in contact with a cathode 4 is referred to as an electron injecting layer 5. Furthermore, when there is a distinct layer between the light emitting layer 1 and the hole injecting layer 3, the distinct layer is referred to as a hole transporting layer 6, and furthermore, when there is a distinct layer between the light emitting layer 1 and the electron injecting layer 5, the distinct layer is referred to as an electron transporting layer 7. It is to be noted that reference numeral 8 denotes a substrate in FIG. 1.
For the low molecular weight-type organic EL elements, films are formed by a vapor deposition method, and multi-layered elements can be thus easily achieved by carrying out vapor deposition while sequentially changing compounds used. On the other hand, for the high molecular weight-type organic EL elements, films are formed with the use of a wet process such as printing or ink-jet printing, and a problem is thus caused which is that the lower layer is dissolved when the upper layer is applied. Therefore, it is difficult to achieve multi-layered high molecular weight-type organic EL elements, as compared with the low molecular weight-type organic EL elements, and it has not been possible to achieve the effect of improving the luminescent efficiency or improving the lifetime.
In order to address this problem, several methods have been ever proposed. One of the methods is a method of using a difference in solubility. For example, there is an element that has a two-layer structure of: a hole-injecting layer of water-soluble polythiophene:poly(styrene sulfonate) (PEDOT:PSS); and a light emitting layer formed with the use of an aromatic organic solvent such as toluene. In this case, the PEDOT:PSS layer is not dissolved in the aromatic solvent such as toluene, and it is thus possible to prepare the two-layer structure.
Patent Literature 2 discloses, in order to overcome such a problem, a method of insolubilizing thin films against solvents by changing the solubility of a compound through the use of polymerization reactions of siloxane compounds, oxetane groups, vinyl groups, etc.
Although these methods for multi-layers are important, there are problems such as the need to remove water remaining in thin films in the case of using the water-soluble PEDOT:PSS, available materials limited for the use of the difference in solubility, and instability of siloxane compounds with moisture in air, and the problem of inadequate element characteristics.
While it is desirable to increase the number of organic layers and separate the functions of the respective layers in order to improve organic EL elements in efficiency and lifetime, there is a need to keep the lower layer from being dissolved in the formation of the upper layer as described above in order to increase the number of organic layers with the use of a wet process which easily forms films even for large area, and approaches have been adopted in which the solubility in a solvent is changed through the use of a polymerization reaction.
Furthermore, for lowering the driving voltages of organic EL elements, attempts have been made to improve the charge transporting performance of a charge transporting compound through the addition of an electron accepting compound to the charge transporting compound, but the performance has not been adequate yet.
On the other hand, the ink compositions which use the materials have problems such as the need for treatment at high temperature for curing and thus difficulty in applying resin substrates, or the need for heating for a long period of time and thus low productivity.